How to drive efficient duckweed production using dairy factory wastewater

In late 2017, we reported on an Irish project, Newtrients, involving the capture of essential nutrients from dairy processing wastewater to grow duckweed as a novel feed protein. Some 13 months on, we hear from the project leader as to the targets met so far.

Newtrients​​ is a collaborative effort that includes scientists working in different disciplines - engineering, microbiology and plant science. The team is located at University College Cork (UCC).

One of the goals of the project is to reduce Ireland’s reliance on soy imports for feed protein. April 2019 will see the initiative reach the mid-way point, with the project given funding for four years. UCC plant scientist and project lead, Professor Marcel Jansen, told FeedNavigator the team has achieved a lot in the initial phase.

“To recap, we work with dairy industry waste and the idea is that we have a two-step process - acidogenic fermentation to produce [biodegradable] bioplastics, removing a lot of the organic material from the dairy processing wastewater [but not the] nitrogen or phosphate, which the duckweed uses, [those are the nutrients that allow us to] create a high protein feed source.” ​

“It sounds simple – the principle idea of using dairy processing waste – but we had to work through quite an elaborate process in order to [determine how to] get the duckweed to grow on that waste. We have now cracked that. We know now what is in that waste, what duckweed needs to grow and what it tolerates. ​

"We have reached a stage where we get very good duckweed growth rates on the waste that we get from a local dairy processing plant.”​

The team eventually ascertained that the required modification to the duckweed cultivation process to get those growth rates was the addition of supplementary calcium:

“It sounds incredibly simple now that I say it, but it [calcium] was a tough one to identify, a needle in a haystack, considering that a plant needs some 20 to 30 nutrients to grow.” ​

The researchers also know how much nitrogen and phosphorus is taken up by duckweed per day. “We have numbers for that now.”​

The team is still working at the lab scale.

“We have upscaled to the 15 liter tank scale now, which is laughable from a dairy processing waste perspective but, for us, it is a major step. We are currently building a 200L unit. In a year and a half, we should be going to 2,500L tank and even that, while massive for us, is still laughable from an industry point of view. But it shows the steps [we are taking] in our upscaling process, with [us going] from 100ml to 15L to 200L and, eventually, to 2,500L.” ​

Indoor cultivation ​

A temperature controlled indoor stacked system, much like a bookshelf structure, using LED lighting, is now envisaged for growing the duckweed; the ultimate idea would be to have an enormous service area that can operate 24/7 over 12 months a year, with commercial production at the scale of an industrial size shed.

Such an indoor production system has replaced the team’s original idea of growing the duckweed outdoors.

"The dairy industry has waste year round, whereas outdoors, duckweed does not grow in the winter months.”​

Dairy companies, he said, also have the means to invest in this kind of high tech indoor duckweed cultivation operation.​

“Also, we have much more scope for automation [in this kind of indoor system].”​

A group in Germany has been doing something similar, he said.

“So we know it is feasible. I think the main driver is that it fits better with industrial practice [than outdoor cultivation]. We can run the system and speed it up or slow it down depending on the production of dairy industry waste, for which there is no storage capacity, no buffer.”​

Indoor cultivation of crops is enabled by the lower cost of LEDs, he stressed. “We have also been inspired, to some extent, by developments in urban farming and hydroponics.”​

The team will build a small-scale indoor stacked shelved unit at UCC this year.

Protein content ​

The researchers are trying to boost the protein content in the duckweed.

“We know that we can get some 15-20% of the dry weight [of the duckweed plant] as protein. Are we happy with that? No, not at all, because we have seen, from the literature, that [the protein content of duckweed] can be somewhere between 30-40%. Using our small-scale system, we want to understand the reasons why we don’t have as high a protein content as is possible; and then we want to try and reach 30-40% [of protein content].”​

Then, it is a matter of harvesting and drying the duckweed, he said.

“At that stage, the project probably comes to an end, though we are talking to one or two feed producers in Ireland about using the material [in trials]. Now, we are a little bit stuck in terms of that, as a feed producer is not interested in receiving just 100 grams of duckweed from us. We need a larger system where we can produce hundreds and hundreds of kilos for a feed company to do feeding trials.”​

Jansen is also involved in a parallel duckweed cultivation project, one that is focused on supplying the aquaculture industry in Ireland. “There is a newly build fish farm in the Irish Midlands (Co Offaly), where all the aquaculture waste is going to duckweed channels. We have one and half hectares of duckweed channels that have been dug out, and they will be populated in the spring. So the duckweed for the necessary feeding trials might come from that project.”​

Duckweed is far too valuable to use as compost, and it makes a poor biofuel, he stressed.

“Its main value is as a feed protein source. That has always been our target sector.”​

The vision then is that as the wastewater is produced at the dairy factories, the indoor cultivation units would be co-located beside those processing plants, given the technical expertise required to run them. The final dried duckweed product would then be sold off to a local feed supplier.​